fix(mcp): keep codegraph_explore loaded in Claude Code and Copilot CLI

[bompus]

Fixes #1696.

Claude Code defers every MCP tool behind its ToolSearch step by default, so a fresh session sees only codegraph_explore's name until the model searches for it. On a default codegraph install setup with no project rule naming the tool, 4 of 6 headless Opus sessions never called it and did the task with Grep/Read; with the schema in context from the first turn, 6 of 6 did (numbers and the reproduction in #1696).

Change

• codegraph_explore carries _meta: { "anthropic/alwaysLoad": true } in src/mcp/tools.ts, and ToolDefinition gains an optional _meta. Claude Code reads the key and keeps the tool's schema loaded from the first prompt. It reaches an existing install on upgrade without touching the user's config; other hosts ignore _meta.
• The Claude Code installer target writes "alwaysLoad": true on the server entry (src/installer/targets/claude.ts), on both the project .mcp.json and the global ~/.claude.json path. Re-running codegraph install adds the key to an entry that predates it. The README's manual ~/.claude.json snippet shows the key.
• The Copilot CLI target writes "deferTools": "never" on its entry (src/installer/targets/copilot-cli.ts). Copilot CLI's tool search defers MCP tools once ~30 tools are connected, and that key is its per-server opt-out (docs). Docs-verified only; the host is not installed here.
• Tests: mcp-tool-annotations.test.ts asserts the _meta on every tools/list surface (the master array, getStaticTools(), ToolHandler.getTools(), and the live server); installer-targets.test.ts covers the key on the Claude project and global paths, the Copilot CLI entry, and the re-run migration for both.
• CHANGELOG entry under Unreleased › Fixes.

Verified

• Headless Claude Code 2.1.261, Opus, --strict-mcp-config with the plain entry: the released 1.6.0 server runs ToolSearch then codegraph_explore; this branch's build runs codegraph_explore alone.
• vitest: 233 passed, 3 skipped. tsc clean.

Not in this PR

OpenCode 2's Code Mode, which puts MCP tools behind a code interface by default. Its per-server opt-out, "codemode": false, only survives on the native v2 entry shape, which the installer does not write yet. Separate issue to follow.

[danusha2345]

Merged into a local integration build of current main (one trivial conflict in targets/claude.ts against #1029's CLAUDE_CONFIG_DIR helper — both functions kept). Verified over stdio: tools/list now returns codegraph_explore with _meta: {"anthropic/alwaysLoad": true}, so an existing install picks it up on upgrade without touching ~/.claude.json. installer-targets and mcp-tool-annotations green; full suite 252 files / 4349 passed.